DC--DC converters are used to convert an input DC voltage to an output DC voltage. In one class of DC--DC converters, known as switching mode converters, the output voltage is determined by the duty cycle of a switch to which the input voltage is applied.
An example of a switching mode DC--DC converter is the complementary synchronous buck converter 10 shown in FIG. 1. A complementary pair of MOSFETs M1 and M2 are connected in series between the input voltage V.sub.in and ground, P-channel MOSFET M1 serving as a series switch and N-channel MOSFET M2 serving as a shunt switch. The common node between MOSFETs M1 and M2 is connected through a low-pass filter including an inductor L1 and a capacitor C1 which deliver the output voltage V.sub.out to the load. V.sub.out is fed back to a pulse width modulation (PWM) control 12, which supplies a PWM signal to the gates of MOSFETs M1 and M2. V.sub.out is determined by the duty cycle of the PWM signal, i.e., in this case the percentage of the time during each cycle that the PWM signal is low, thereby turning P-channel MOSFET M1 on. PWM control 12 is controlled by the feedback path to maintain V.sub.out at a desired level.
DC--DC converters are available in a wide variety of topologies. FIG. 2 shows a totem pole N-channel synchronous buck converter 20, in which V.sub.out is determined by the duty cycle of high segment of the PWM signal applied to the gate of N-channel MOSFET M3. PWM control 22 supplies time delayed signals to the respective gates of MOSFETs M3 and M4 so as to prevent current "shoot through" from V.sub.in to ground. FIG. 3 shows a boost converter 30 which includes an N-channel MOSFET M5 and a Schottky diode 32.
A common feature of the converters shown in FIGS. 1-3, as well as numerous other converter topologies, is that one or more power MOSFET switches are used to control the transfer of energy from an energy source, here represented by V.sub.in, into at least two reactive energy storage elements, namely an inductor and a capacitor. These energy storage elements then retransfer the stored energy, when required, into the load. By monitoring V.sub.out and by either controlling the pulse width of the signal which controls the MOSFET switches (assuming that the converter is operating at a fixed frequency), or adjusting the switching frequency (while holding the on-time of the switches constant), a constant V.sub.out can be maintained, despite changes in V.sub.in or the current demands of the load.
Of the various switching mode converter topologies and control schemes, fixed frequency converters provide a predictable noise spectrum. A predictable noise spectrum is particularly advantageous in communication products, such as cellular phones, since shifting noise spectra can interfere with information transfer in the broadcast band. With a fixed clock period, the energy transfer is a function of the switch on-time (or pulse width), which is modulated to compensate for an energy drain or a voltage build-up at the output of the converter.
Most converters, in their essential configuration, include a PWM control circuit, an inductor, a capacitor, and two MOSFET switches (or one MOSFET switch and a Schottky diode). Ideally, every element transfers power without loss. In reality, of course, some power is lost in every element. The IC control circuit, for example, draws power to operate internal amplifier, voltage reference, comparator, and clock circuits. The inductor loses power to the resistance of its coil and to the material used as its magnetic core. Even the capacitor has a series resistance component which absorbs energy.
In practice, however, most of the power in a converter is lost in the power MOSFET that is used as the series switch and in the power MOSFET or Schottky diode that is used as the shunt switch or rectifier. These losses can be divided into four categories:
1. Conduction losses which arise from the MOSFETs' internal resistance, represented as I.sup.2 R.multidot.D, where I is the current through the switch, R is the on-resistance of the switch, and D is the percentage of the time that the switch is on.
2. Gate drive losses, or the power lost charging and discharging the MOSFETs' gate capacitance, represented as Q.sub.g .multidot.V.sub.gs .multidot.f, where Q.sub.g is the charge which accumulates on the gate, V.sub.gs is the gate-to-source voltage, and f is the frequency at which the switch is opened and closed.
3. Output capacitive losses, or the power lost charging and discharging the drain capacitance of the MOSFET switch, represented as C.sub.o .multidot.V.sub.ds .multidot.f.
4. Crossover losses, or losses which occur during the switching transitions of the MOSFETs, as a result of the simultaneous presence of a current through and a voltage across a MOSFET, represented as I.sub.on .multidot.V.sub.ds .multidot..differential.t, where I.sub.on is the current through the MOSFET during the switching transition and .differential.t is the duration of the switching transition.
The conduction losses are strongly dependent on the current and on-resistance while the gate drive and output capacitive losses are strongly dependent on the switching frequency. At low frequencies, particularly below 100 kHz, only the conduction losses need to be considered when calculating the efficiency of the converter. At higher frequencies, particularly frequencies approaching 1 MHz, the capacitive losses become significant. V.sub.in and V.sub.out affect all of the energy loss terms. In high voltage converters the output capacitance term can be dominant. In low voltage applications such as computers and battery powered circuits, however, particularly those in which V.sub.in is less than 8 volts, the output capacitance term is negligible. The two dominant terms are then the gate drive and conduction losses, and the power loss can be approximated by the following equation. EQU P.sub.loss =Q.sub.g (V.sub.gs).multidot.V.sub.gs .multidot.f+I.sup.2 .multidot.R.sub.ds (V.sub.gs).multidot.(t.sub.on (V.sub.in)/T)
An increase in the gate drive V.sub.gs reduces R.sub.ds and conduction losses but increases gate drive capacitance losses. The frequency f and the load current I are weighting factors which determine which term is dominant. At higher frequencies, the gate drive capacitance loss becomes significant for all light load conditions.
There is therefore a need for a MOSFET which provides low gate capacitive losses during light load conditions while providing low conduction losses during normal load conditions, without relying on frequency shifting or burst mode techniques.